In add_parents_to_list, if any parent of a revision had already been
SEEN, the current code would continue with the next parent, skipping
the test for --first-parent. This patch inverts the test for SEEN so
that the test for --first-parent is always performed.
Signed-off-by: Lars Hjemli <hjemli@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>